Cutting pcb lines and soldering on your own component? That will be hard if
not impossible to make work; the antenna is actually part of the pcb (it
still has connecting lines, of course).
https://www.raspberrypi.org/magpi/pi-zero-w-wireless-antenna-design/

No, there's a resistor (probably 0ohm) that connects it and you can
move it to instead connect with a footprint left for an antenna
connector:
https://www.briandorey.com/post/raspberry-pi-zero-w-external-antenna-mod

60MBps is the maximum signal bit rate, yes... But that ignores overhead
in USB control signaling (turn around between polling the bus to find
devices with data to transfer, requesting the transfer, length of packet
transferred -- shorter packets will involve more overhead -- ack/nak of
packet, encode/decode time [checksum or CRC, and USB3 at least uses an
encoding that takes 10bits per byte, to ensure state changes occur often
enough to keep the sender/receiver in sync -- no runs of eight 0 bits, for
example]...).

The effective max is around 30MBps -- and that probably assumes a
single client per host controller. With more than one device on the bus,
the host will have to poll each device, then determine which gets to
transfer data.

Clock speed is pretty immaterial, it is a dedicated router chipset, with
gigabit Ethernet, a/b/c/g/n/ac WiFi, and is vastly better as a router
than a general purpose Pi Zero W with everything going over a single USB
bus.

Well you wont find anything better at doing WiFi routing than a WiFi router.

I'll admit that I've been lazy and haven't compared the FCC data for
USB WiFi adapters yet. But I've seen that there are some designed for
longer range, so I'm hopeful that there will be something equivalent
to the transmission power of my existing router, if the Pi ZeroW's
onboard WiFi doesn't prove powerful/reliable enough.